This invention relates to an all-polymeric ultraviolet reflecting film, and more particularly to a reflector which is substantially transparent to visible and near infrared wavelengths while reflecting a substantial portion of solar ultraviolet wavelengths.
A major problem exists in this and other countries because of the ever-increasing production of hazardous water-borne wastes as by-products of industry. Nearly 240 million tons of hazardous wastes are discharged each year in this country into sewers, streams, ponds, and rivers. Other hazardous wastes leach into ground water supplies from landfills or damaged underground storage tanks.
While some of the wastes can be contained at the source of their production, once the wastes materials enter large water sources, their concentration becomes diluted. Still, the level of concentration of the wastes pose health hazards to the general population. Because the volumes of water which must be decontaminated are so large and the concentrations of wastes so dilute, purification becomes an enormous undertaking.
One method of treating large volumes of dilute liquid hazardous wastes has been solar detoxification. Solar detoxification utilizes the energy of the sun, specifically the ultraviolet portion of the solar spectrum, to degrade organic hazardous wastes in water. Contaminated waste is treated using a combination of solar radiation and light-activated catalysts. A wide variety of organic pollutants can be broken down into carbon dioxide, water, and simple mineral acids which then can be easily neutralized using this process. Such wastes include pesticides, industrial solvents, preservatives, dyes, and hydrocarbon fuels.
Typically, a solar detoxification system includes a series of solar reflectors which reflect the sunlight onto clear glass tubes carrying the liquid waste stream. Catalyst is introduced into the tubes either by mixing with the waste stream or by fixing the catalyst onto a porous lattice-like structure in the tubes through which the waste stream flows. Ultraviolet light wavelengths of between 300 to 400 nm are needed for the catalytic degradation process to proceed. Thus, because only about 3-4% of the solar energy spectrum, is within this range, very efficient reflectors are required. Further, as excessive solar heating of the stream may be in many instances undesirable, such reflectors ideally are transparent to visible and near infrared wavelengths.
Because of the size of such reflectors and the need for solar exposure, materials for use in such reflectors need to have outdoor durability. Further, the reflectors should have specular reflectance characteristics and have a reflectance which is relatively insensitive to the angle of incidence of the sun's rays. Ultraviolet reflecting films have been fabricated from bare metals such as electropolished aluminum, coated films such as aluminum-coated polymer films, or multilayer metal oxide coatings. However, each of these classes of materials suffers from one or more deficiencies.
Polished metal surfaces are of questionable outdoor durability due to the formation of oxides thereon from the presence of moisture and air pollutants. Metal surfaces also reflect visible and solar infrared wavelengths and may cause unwanted heating in certain applications. While protective layers of materials may be applied, that increases manufacturing costs and may adversely affect the specular reflecting characteristics of the metal. Metallized polymer films have been developed for indoor lighting applications. However, such metal-coated polymers are not recommended for outdoor use. Multilayer metal oxide coatings require repetitive layer depositions using sputtering or chemical vapor deposition techniques. The costs to produce multilayer metal oxide films are presently prohibitive for the large surface areas needed for solar detoxification reflectors.
There is also a need for ultraviolet light reflecting films in other technologies. For example, UV mirrors are used in the fields of medical imaging, astronomical telescopes, microscopy, chemical reactions which use ultraviolet light as a curing mechanism, indoor lighting, microlithography, industrial micro-machining, and ultraviolet laser reflection. It would be desirable to be able to have low cost, highly UV reflective products for use in those technologies as well as in solar detoxification.
Reflective multilayer articles of polymers are known, as are methods and apparatuses for making such articles. For example, such multilayered articles may be prepared utilizing multilayer coextrusion devices as described in commonly-assigned U.S. Pat. Nos. 3,773,882 and 3,884,606 to Schrenk. Such devices are capable of simultaneously extruding diverse thermoplastic polymeric materials in substantially uniform layer thicknesses. The number of layers may be multiplied by the use of a device as described in commonly-assigned U.S. Pat. No. 3,759,647 to Schrenk et al.
Alfrey, Jr. et al, U.S. Pat. No. 3,711,176, teach a multilayered highly reflective thermoplastic body fabricated using optically thin film techniques. That is, the reflective optically thin film layers of Alfrey, Jr. et al relied on the constructive interference of light to produce reflected visible, ultraviolet, or infrared portions of the electromagnetic spectrum. Such reflective optically thin films have found use in decorative items because of the iridescent reflective qualities of the film. The patentees also describe an ultraviolet blocking film which is transparent to visible light used for protective packaging for foods.
Im and Schrenk, "Coextruded Microlayer Film and Sheet", Journal of Plastic Film & Sheeting, vol. 4, pp. 104-115 (April 1988), describe a multilayer ultraviolet reflecting film of polycarbonate and polypropylene used to block ultraviolet radiation from reaching packaged foods. However, like the Alfrey et al UV reflecting film, the film is designed to block ultraviolet radiation from reaching the foods packaged in the film, and the polymers utilized in fact absorb significant amounts of ultraviolet radiation.
Accordingly, the need still exists in the art for low cost, weather resistant, ultraviolet light reflecting (but non-ultraviolet absorbing) films which can be used for a variety of purposes. The need also exists for such materials additionally being substantially transparent to visible and infrared light.